1. Field of the Invention
This invention relates to a system for use in computer assisted surgery. More specifically, the invention relates to a system for providing visual feedback regarding surgical tool positioning with respect to fluoroscopic images of a body part during an orthopaedic procedure.
The invention also relates to a system for providing the surgeon with improved visual feedback for the positioning of one surgical tool with respect to another surgical tool or implant.
2. Description of the Related Art
Orthopaedic procedures generally involve the fixation of a screw, plate, prosthetic component, or other implant to the bone of a patient. Typically the bone into which the implant is inserted or affixed is only partially exposed to the surgeon""s vision. In order to align the implant with respect to the unexposed bone, some sort of imaging modality is required (preoperative x-rays, preoperative CT scans, or intraoperative x-rays using a C-arm fluoroscope). However, these images can be very difficult to correlate to the patient""s anatomy in a useful manner. The field of image guided surgery is concerned with the use of computer technology to present these images to the surgeon in a manner that makes them more relevant and useful.
In the case of intertrochanteric hip fractures, the treatment of choice is the insertion of a lag compression screw. The first step in this procedure is the insertion of a guide pin along the intended trajectory of the screw from the lateral femur through the center the femoral head. This has been traditionally performed with repeated images from a C-arm, allowing the surgeon to monitor the alignment and progress of the guide pin insertion. Because x-ray images provide information in only two dimensions, two separate images taken from different positions are required to demonstrate the correct positioning of the guide pin in three dimensions. In practice, this means that the C-arm must be repositioned each time an updated set of images is acquired. Not only does this add to the duration of surgery, but during this time the surgical instrument visible in the existing image may move. Thus it is not guaranteed that two orthogonal images will represent the current pose of the surgical tool. (An object""s pose may be defined as its position in space and, to the extent known or calculable, its orientation.) Further, the images that are acquired by the C-arm do not represent linear projections of the anatomy. The image captured by the image intensifier and camera unit of the C-arm is subject to distortions due to both the geometry of the image intensifier and the effect of magnetic fields (including Earth""s magnetic field) on its internal electron beam. These cause a xe2x80x9cwarpingxe2x80x9d of the image and lead to straight objects appearing curved in the x-ray images. Further, the degree of distortion varies with respect to several factors including C-arm orientation, image intensifier shielding and size and proximity of ferrous objects. Other factors, such as rigidity of the source/receiver connecting structure and operating temperature, as well as magnetic fields, induce a translational offset to the image.
This inability to obtain accurate and linear images, simultaneously in two views, may lead the surgeon to insert the guide pin along a path other than the intended one. These misplaced attempts can add significantly to the duration of the surgery and the amount of radiation exposure to OR personnel as well as compromise of the bone stock. Further, the risks of a prolonged procedure and the difficulty of inserting a guide pin near the hole from a previous failed attempt may lead the surgeon to accept a pin position that is suboptimal. A serious complication, the xe2x80x9ccutting outxe2x80x9d of the screw through the femoral head into the hip joint, has been linked in numerous studies to poor placement of the screw.
Several image guided systems have been proposed to deal with the problems of this and similar surgeries. U.S. Pat No. 5,517,990, Kalfas, et. al., May 21, 1996, describes an image guided surgical system that is similar in concept to the majority of systems currently in use. This system uses sonic tracking of a probe to navigate CT data of the patient""s head or spine. However, CT scans are not indicated for most orthopaedic trauma procedures and would add significantly to the cost of treatment if obtained. Further, CT scans must be registered to the bony anatomy (i.e., a mathematical relationship must be found between the coordinate frames of the CT scan and of the bone). This requires an intraoperative step in which a probe is used to sample the positions of landmarks on the bone as these same points are selected in the imaging data. (Alternatively, small radiopaque markers may be used as landmarks.) Such systems and their complicated user interfaces are often found by surgeons to be time consuming and difficult to use.
Another image guided system has been described in U.S. Pat. No. 5,772,594, Barrick, Jun. 30, 1998. This system displays the pose of a surgical tool over intraoperative fluoroscopic images during hip screw placement. This system, however, requires that the bone be registered to the images by finding small, different shaped, radiopaque markers. This introduces extra steps to the process and may negate the potential time savings. Also, no method is described for the correction of the nonlinearities present in the C-arm images.
Another solution for the difficulties in hip screw placement is proposed by Phillips, et. al. They describe a fluoroscopic system wherein image processing techniques are used to identify a path for the guide pin. The surgeon then implements this by aligning the tool, connected to a passive manipulator, until crosshairs on the display align. The drawback of this system is that it uses the surgeon as an assistant to implement its plan instead of providing improved information to the surgeon with which to plan and execute the procedure.
Another application for the system proposed by Phillips, et. al. is the insertion of a screw through a transverse hole in the distal end of an intramedullary (IM) rod that has been inserted down the central canal of a fractured femur. In order to insert this locking screw, a hole is drilled in the bone exactly at the location of the transverse hole with the same orientation. Currently the surgeon aligns the C-arm with the transverse holes so that they appear as xe2x80x9cperfect circlesxe2x80x9d in the images. The surgeon then uses repeated images to align the tip of the drill with the center of the hole while using the C-arm source and receiver as external reference points to correctly orient the drill. This procedure involves numerous x-ray images and often requires several attempts before the screw hole is acceptably placed.
The biggest drawback with using a C-arm to position a drill for IM rod screw insertion is the difficulty encountered in achieving the accurate orientation of the drill in the axial plane. External jigs, attached to the exposed proximal end of the IM rod, have been proposed to assist in the placement of the distal screw holes, but these are unable to account for flex of the IM rod in the bone and therefore are not very useful. The system proposed by Phillips, et. al. extracts features from fluoroscopic images of the inserted IM rod and uses image processing techniques to calculate the trajectory required to pass a drill through the hole. The surgeon then moves a drill guide attached to a passive manipulator until the proper position is achieved and then drills the hole. Again, the drawback of this system is that it uses the surgeon as an assistant in implementing its plan instead of providing improved information to the surgeon with which to plan and execute the procedure.
A similar difficulty encountered by surgeons is the accurate placement of a hole or guide pin through an irregularly shaped or partially obscured bone when fluoroscopic guidance is not used. For example, when drilling holes through the patella for tendon or fracture repairs or the calcaneous for fracture fixation, it may be difficult to correctly align the drill with the intended exit point. The system described in U.S. Pat. No. 5,305,203, Raab, Apr. 19, 1994, includes a means for implementing a previously specified drill trajectory as part of a menu driven surgical system. A drawback of this system is the sequential nature of the indication of the entry point, the indication of the exit point and the implementation of the trajectory by a single passive manipulator arm.
Many of these systems often suffer from a lack of readiness for the operating room. As academic or conceptual systems they do not always address practical considerations. Many systems introduce extra equipment and operative steps to the surgical procedures that prolong the surgery and require significant training. Further, most of the systems do not address the issues of sterility, error checking and safety, and unwanted motion of the body part to be operated upon.
Most systems require input from the surgeon in order to specify data or alter program flow. Many systems rely on a non-sterile assistant to enter data at a keyboard or with a mouse, but this is inefficient and risks miscommunication. A sterilized or draped input device introduced into the surgical field may be difficult to use and distracting for the surgeon. Visarius describes an input scheme in which the surgeon points to fields on a tracked, sterile xe2x80x9cvirtual keyboardxe2x80x9d with the surgical tool. The input scheme described in U.S. Pat. No. 5,230,623, Guthrie, Jul. 27, 1993 uses the surgical tool pointing to an area in space to move a mouse cursor on the screen via an xe2x80x9cimaginary mathematical correspondencexe2x80x9d. Both, however, require the splitting of the surgeon""s attention between the display screen in one location and the surgical tool in another as well as the removal of the tool from the surgical site for use elsewhere as an input device.
In order that any motion of the body part which is being operated upon not affect the accurate superposition of the tool on the image data, many systems use a dynamic reference frame. U.S. Pat. No. 5,383,454, Bucholz, Jan. 24, 1995, describes the measurement of all surgical tool poses relative to a ring or to markers attached to the patient""s head. This allows the registration between the three dimensional image data and the patient""s skull, as well as the accurate positioning of the tool relative to the head, to be maintained despite motion of the head. However, some surgeries, especially orthopaedic trauma procedures, involve multiple body parts (e.g., bone fragments, soft tissue). While not freely mobile, these untracked body parts may experience significant motion if associated structures are moved excessively.
Accordingly one feature of our invention is an image guided surgical system that provides a surgeon with improved visualization of the relationship between surgical tools and the involved body part, by accurately superimposing representations of tools being used in the surgical field over the images of the body part such that real-time tool position feedback is provided to the surgeon, and which comprises a controller, display device, localizing device, and surgical tools with localizing emitters and which receives image data from a fluoroscopic x-ray device, preferably a C-arm.
A related feature of the invention is the use of a fluoroscopic x-ray device employing a digital flat-panel x-ray imager comprised of a regular array of sensors to convert incident x-ray energy to a computer readable image, and has the advantage of improving the accuracy with which the imaging chain can be modeled and permits the fluoroscopic x-ray device to be used in any orientation without being affected by distortions due to local magnetic fields.
Another feature of the invention is the preoperative determination of imaging model parameters, including conic projection parameters and mapping parameters, that closely model the imaging chain of the x-ray device, and are determined by a process employing a radiopaque calibration grid placed between the x-ray source and x-ray receiver.
A related feature of the invention is the determination of the conic projection model parameters for a plurality of orientations of the fluoroscopic x-ray device.
Another feature of the invention is the specification of a conic projection model, including its pose, and the calculation of conic projection parameters through interpolation of preoperatively determined pose-specific calibration data, said conic projection model simulating the way an acquired image is formed on the fluoroscopic x-ray device""s input surface during surgery, such that points in space can be mapped to image space in an accurate simulation of the imaging chain of the fluoroscopic device, and has the advantage of permitting accurate superposition of graphic representations of surgical objects on fluoroscopic images of a body part without the requirement for registration of the body part itself.
Another feature of the invention is a surgical tool outfitted with a plurality of localizing emitters such that its pose can be continuously measured by a localizing device, and that has an associated three dimensional graphics model, such that points comprising the model can be assigned a position in space, transformed into image space using the conic projection model and mapping equations corresponding to an acquired image, and then superimposed on that image, thereby producing a graphic representation of the surgical tool that duplicates the movement of the tool in real time.
Another feature of the invention is a device outfitted with a plurality of localizing emitters such that its pose can be continuously measured by a localizing device, and that relatively small translational and rotational motions of the body part to which the device is attached can be determined and a compensatory adjustment made to the tool representation on the image to preserve the accurate relationship between the tool representation and the image of the body part, while larger motions raise a warning to the surgeon.
Still another feature of the invention is software that assists the surgeon in verifying the accurate superposition of a representation of a tool over an x-ray image of that tool whose pose is recorded at the time of image acquisition and which is generated in the same manner as the real-time representation, but is retained on the screen until the surgeon has had time to visually determine its accurate superposition over the silhouette of the surgical tool in the x-ray image.
Still another feature of the invention is software that provides a direct and convenient user interface by performing specified actions when the intersection of the trajectory of the surgical tool with the plane of the display device screen occurs in specified screen fields and an activation criterion, such as continued field selection or the press of a button, is satisfied.
Still another feature of the invention is a sterile transparent drape that allows the C-arm to be used in a sterile environment, and that includes a portion similar in size and shape to the housing that contains the C-arm localizing emitters, such that the drape fits flat and flush against the localizing emitters, so that their positions may be accurately determined by a localizing device.
A feature of an alternative embodiment is a plurality of localizing device sensor units that permit the localization of emitters on either side of an obstruction such as a sterile drape, and can further be reoriented to provide a wider field of view or a more accurate, redundant, narrower field of view, and that further comprises controller software to coordinate the activation of localizer sensor units and localizing emitters and to select the appropriate position data from the multiple localizer sensor units to calculate the most accurate emitter location, and that further comprises a registration object containing localizing emitters and a registration procedure for determining the pose of one localizing device sensor relative to another.
A feature of an alternative embodiment is a computer assisted surgical system that assists a surgeon in positioning a first surgical tool with respect to a second surgical tool by providing a continuously updated picture of the relative poses of the two tools, and which further provides for the software selection of the projection method and plane that generates this picture, and which comprises a controller, display device, localizing device, and surgical tools with localizing emitters.
A feature of an alternative embodiment is an image guided surgical system that assists the surgeon by providing information regarding the pose of a first surgical tool relative to images of a body part and relative to a second surgical tool, by accurately superimposing a representation of the first tool over the images of the body part such that real-time tool pose feedback is provided to the surgeon, and by providing a continuously updated picture of the relative poses of the two tools, and which further provides for the software selection of the projection method and plane that generates this picture, said system comprising a controller, display device, localizing device, and two surgical tools with localizing emitters and which receives image data from a fluoroscopic x-ray device, preferably a C-arm.
These and other features and advantages are, in the present invention, embodied in an improved system for assisting a surgeon in positioning a surgical tool, optionally connected to an implant, with respect to a body part. The system uses a one-time calibration process involving a calibration grid, containing markers visible in x-ray images, to determine pose specific imaging model parameters which are stored in the controller. As images are acquired during surgery, the pose of the fluoroscopic x-ray device containing localizing emitters is measured by a localizing device. Imaging model parameters are interpolated from the calibration data to simulate the fluoroscope""s imaging chain for the pose at which the image was acquired. The poses of surgical tools, containing localizing emitters, are continuously measured by a localizing device. The points of a three dimensional graphic model representing the surgical tool in its pose, are transformed by the imaging model to develop a two dimensional graphic representation of the tool in image space that is superimposed on the corresponding x-ray image. The modeling of the fluoroscopic device for each acquired x-ray image has the advantage of permitting the accurate projection of surgical tool representations on fluoroscopic images of a body part without the requirement for registration of the body part itself.
An feature of the system is the use of a digital flat-panel x-ray imaging device as the source of x-ray images. Such a device provides, for a wide range of operating conditions, high spatial linearity between the x-ray pattern striking its input surface and the resultant electronic image generated. The use of a digital x-ray imager in an image guided system allows for simplified and accurate modeling of the imaging chain. Such a system, which includes surgical tools tracked by a localizing device, is able to provide accurate real-time feedback to the surgeon of the tool poses relative to the involved anatomy.
Other aspects of the invention relate to its reliability and ease of use. A software feature causes the persistence of the representation of a surgical tool on the screen in its pose at the time an image was acquired. This allows the surgeon to verify the system""s accuracy by comparing the superimposed tool representation to the actual x-ray image of the tool. The means for providing the surgeon with time to determine the accuracy of the system may be a timer of predetermined value or the surgeon may control this feature via a button press, a keyboard command, or even a specific tool motion.
Another software feature, intended to provide the surgeon with an interface to the system, calculates the location of the intersection of the surgical tool""s trajectory with the plane of the display screen. If this intersection falls within previously defined areas of the screen, and an activation criteria is satisfied (such as continued selection for a specified period of time), an associated software action is performed.
Another feature is the use of multiple localizing device sensor units to determine pose data of surgical objects either in a wide field of view, more accurately in a narrow field of view, or when the field of view of a single sensor is obstructed, as by a surgical drape.
Another aspect of the invention is the use of the system without imaging data to assist the surgeon in positioning one surgical tool with respect to another. A first surgical tool, containing localizing emitters, is held in a pose with respect to the body part. A second surgical tool, also containing localizing emitters, is held relative to the first surgical tool. A localizing device determines the poses of both tools. A picture plane is chosen in space, and representations of one or more tools are projected onto the picture plane. The surgeon then orients the surgical tools based on the continuous feedback of their representations on the display screen.
Still another aspect of the invention is the use of the system simultaneously both with a representation of a surgical tool superimposed on x-ray imaging data, and in a picture showing the relationship to a representation of a second tool.